This invention relates to a method of manufacturing a thermal head used for the purpose of heating thermally sensitive recording paper in facsimile apparatus, printers etc.
There are known thermal heads of the type comprising a plurality of electrode leads disposed alternately on both sides of an electrically insulating substrate and a ribbonshaped heating resistor bridging the electrode leads. During thermal recording, recording pulses are selectively applied to the electrode leads to generate heat from elements of the heating resistor interposed between the particular electrode leads. This heat is used to record visually information in accordance with the recording pulses on a section of thermally sensitive recording paper fed in opposed contact relationship to the thermal head.
One of the conventional methods of manufacturing such a thermal head has comprised the steps of screen printing a thick film paste of an electrically conductive material in predetermined regions on the surface of an electrically insulating substrate and baking the paste to form electrode leads. Then a thick film paste of electrically resistive material is printed into a ribbon bridging the electrode leads on the surface of the substrate through a stainless steel gauze or a screen having a predetermined width and baked at a predetermined temperature to form a heating resistor. Since the electrode leads are about a few micrometers thick, the heating resistor formed on the upper surfaces of the electrode leads and on the surface of the substrate has an irregular surface but not a uniformly flat surface. This has resulted in the unstable contact of the heating resistor with thermally sensitive recording paper. In other words, serious disadvantages have occured in that recorded dots have been uneven in density and more or less different in size from one another because recording dots are formed of the elements of the heating resistor interposed between the electrode leads and also the electric power required for the recording increases due to the deterioration of the thermal response of the thermal head.
Conventional methods of manufacturing the thick film type thermal head have used, as the screen, metal gauze with fine meshes formed of a fine stainless steel wire. In order to print the electrode leads or heating resistor in a predetermined pattern on the surface of the electrically insulating substrate, the screen has had a corresponding pattern formed thereon by a baking process. However, the dimension of the meshes and the diameter of the wire have lower limits. Therefore it has been practically impossible to form the leads and resistor in very fine patterns. Also since the paste of the electrically conductive or resistive material to be printed is passed through the fine meshes of the screen, the viscosity thereof should range from ten thousand to a hundred thousand centipoises. This has resulted in the blurring and flagging of the paste printed on the substrate. Therefore the resulting pattern has deteriorated in accuracy and accordingly the quality of the reproduced material has deteriorated.
Accordingly it is an object of the present invention to provide a new and improved method of manufacturing a thermal head by which heating resistors can be consistently formed in a predetermined configuration and the resulting resolution can be improved.
It is another object of the present invention to provide a new and improved method of manufacturing a thermal head including heating resistors which are small in size.
It is still another object of the present invention to provide a new and improved method of manufacturing a thermal head including heating resistors having improved flatness of the surface and which can effect high speed recording with reduced electric power.
It is a different object of the present invention to improve the thermal response of the thermal heads.